The invention relates to a process for the thermal cracking of residual hydrocarbon oils.
Residual hydrocarbon oils may be obtained by distillation at atmospheric pressure of a crude mineral oil, producing straight run distillate fractions and a residual oil, also called "long residue". The long residue is usually distilled at sub-atmospheric pressure to yield one or more so called "vacuum distillates" and a residual oil, also called "short residue". Both residual oils and also other residual oils such as those obtained from tar sands and shale oils have been subject of much research aiming at conversion thereof into more valuable products.
The article in "Wissenschaft und Technik, Erdoel und Kohle-Erdgas-Petrochemie vereinigt mit Brennstoff-Chemie", 36, Oct. 1983, 457-461 concerns thermal cracking of residual hydrocarbon oils in the presence of hydrogen. A tar sand heavy oil is cracked in this manner with formation of cracked products from which a gaseous fraction containing hydrocarbons having one to four carbon atoms per molecule and a liquid residual fraction were separated. The liquid residual fraction contained 70% by weight of distillate with a final boiling point of 592.degree. C. The experiments were carried out by preheating the tar sand heavy oil to a temperature of 375.degree. C. and the hydrogen to a temperature of 12OO.degree. C. and introducing the preheated material into a reaction vessel, the temperature of the hydrogen being sufficiently high to provide the heat required for the thermal cracking.
Japanese patent application publication No. 62-96589 concerns a process in which a mixture of a heavy hydrocarbon oil, hydrogen and carbonaceous fine particles is thermally cracked, the cracked products are separated into a gas, a light oil, a middle oil and a cracked residue, the cracked residue is deasphalted with formation of a deasphalted oil and a fraction relatively rich in asphaltenes, the hydrocarbon fraction relatively rich in asphaltenes is gasified in the presence of oxygen and steam with formation of synthesis gas, carbonaceous fine particles are separated from the synthesis gas and the separated particles are recycled to the thermal cracking. The presence of hydrogen in the thermal cracking zone reduces problems brought about by formation of carbonaceous products in thermal cracking and yields oil having a high stability and a low olefin content. In this known process the material to be thermally cracked is indirectly heated to the cracking temperature, that is to say via a wall through which heat is transmitted. A disadvantage thereof is the possibility of a gradual buildup of carbonaceous deposits on the inner wall by the mixture to be thermally cracked, resulting in a reduced on-stream time of the furnace in which heating takes place. This disadvantage is particularly important in those cases where high conversions of the heavy hydrocarbon oil are aimed at. Another disadvantage is that particles of metals and ash which are usually present in synthesis gas are not removed therefrom. Consequently, metals and ash will be present in increasing concentration.
It is an object of the present invention to avoid the disadvantages of indirect heating of the residual hydrocarbon oil to the cracking temperature.
Another object is to avoid concentration of particles of metals and ash in the process.
A further object is to use hydrogen in an easily available form.